1. Field of the Invention
The present invention relates to a laser apparatus and, more specifically, to a mode-locked type laser apparatus.
2. Description of the Background Art
As the total capacity of transmission has been rapidly increasing recently, in the field of optical fibers, increase of the transmission rate per unit wavelength has been desired as an approach for practical implementation of ultra high speed/ultra large capacity optical transmission. For this purpose, in order to realize optical transmission at a rate of exceeding 100 Gbps per unit wavelength in the near future, mode-locking semiconductor laser diodes (MLLD) have been studied. The mode-locked state includes two different types, that is, amplitude modulation mode locking (AMML) and phase (frequency) modulation mode locking (FMML). In the AMML, all modes oscillate in-phase, and therefore a high energy optical pulse can be obtained. In the FMML, of the oscillating modes, some are oscillating out-of-phase, and therefore, a high energy optical pulse cannot be obtained. Therefore, laser oscillation of AMML is desired in order to obtain a high energy optical pulse. In order to realize ultra high speed and ultra large capacity optical transmission, no matter what repetition frequency is necessary, an AMML laser device that operates at that frequency is desired.
One example of the study of MLLD is disclosed in xe2x80x9cAll-Optical Signal Processing with Mode-Locked Semiconductor Lasersxe2x80x9d, Yokoyama et. al., Proceedings of a symposium by The Institute of Electronics Information and Communication Engineers. The MLLD described by Yokoyama et. al. is adapted to have a saturable absorption effect within the device, whereby basic optical signal processing function as a coherent pulse light source, optical clock extraction, optical gate, as well as optical 3R identification reproduction (3R: retiming, reshaping, regenerating) and optical time division demultiplexing (optical DEMUX) can be attained by a simple arrangement. In the MLLD technique by Yokoyama et. al., mentioned above, however, a saturable absorption section is necessary within the laser cavity. Therefore, an operational instability, such as hysteresis of the light-current characteristic, is a serious problem for the device. Further, the threshold current is high undesirably.
Therefore, an object of the present invention is to provide a laser apparatus not requiring the saturable absorption section and in which modes all oscillate in a mode-locked manner with constant phase differences regardless of the oscillating condition, that is, a laser apparatus that oscillates in AMML.
The above described object of the present invention is attained by the laser apparatus by the present invention, which includes a resonator and a gain section excitation means wherein the resonator includes a gain section in which population inversion is attained by at least one method selected from the group consisting of optical excitation and current injection, non-gain sections in which gain with respect to the laser oscillating light beam is not positive, and two reflection mirrors. The gain section is arranged at a central portion along the optical axis of the resonator, to have the optical path length approximately one half that of the resonator. The non-gain sections are arranged on both sides of the gain section along the optical axis of the resonator. The two reflection mirrors are arranged further outside of the non-gain sections along the optical axis of the resonator. The gain section excitation means is for retaining the excited state of the gain section.
By the above described arrangement, it becomes possible to obtain AMML oscillation even in a laser apparatus having such a resonator length that causes FMML oscillation when the inner portion as a whole of the resonator is used as the gain section.
In the above described invention, preferably, the gain section includes a semiconductor, and the non-gain section includes a dielectric. Alternatively, the gain section includes a semiconductor and the non-gain section includes a semiconductor. By this arrangement, even in a semiconductor laser device, an arrangement that can attain AMML oscillation can be realized easily.
In the above described invention, preferably, a non-gain section electrode is provided, which performs carrier injection to the non-gain section or application of a reverse bias to the non-gain sections. In this arrangement, it becomes possible to inject carriers to an optical wave guide layer by using the non-gain section electrode and, as a result, index of refraction within the resonator changes because of the plasma effect of free carriers, controlling substantial optical path length. Alternatively, by applying the reverse bias, the substantial optical path length can be controlled. Accordingly, repetition frequency of the mode locked optical pulse can be controlled.
In the above described invention, preferably, at least one of the reflection mirrors is a distributed Bragg reflector. By this arrangement, it becomes possible to select the oscillation wavelength of the mode locked laser, and expansion of the oscillation spectrum can be limited.
In the above described invention, preferably, a distributed Bragg reflector electrode is provided, for changing reflection spectrum by current injection or reverse bias application to a periodic structure of the distributed Bragg reflector. When this arrangement is employed, it becomes possible to change the reflection spectrum of the distributed Bragg reflector and to change the wavelength of the mode locked optical pulse.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.